Human hearing and balance impairments are often due to death of sensory hair cells. These cells are sensitive to death induced by noise exposure, aging, and some therapeutic drugs, including the aminoglycoside antibiotics and the antineoplastic agent cisplatin. The goal of this research program is to understand the molecular mechanisms underlying hair cell death and survival. Understanding these cellular mechanisms in detail will be critical for the design of therapies aimed at preventing hearing loss. Induction of heat shock proteins (Hsps) in response to cellular stress is a ubiquitous and highly-conserved response that can significantly inhibit apoptosis in many systems. Our data indicate that Hsp70 induction is a critical stress response in the inner ear that can promote survival of hair cells exposed to major stressors. The mechanism(s) underlying the protective effect of Hsp70 against hair cell death are not known. Our data indicate that Hsp70 induction occurs primarily in supporting cells with little induction in hair cells, suggesting that supporting cells mediate the protective effect of Hsp70 against hair cell death. We hypothesize that Hsp70 is secreted by supporting cells and internalized by hair cells, where it acts as both a molecular chaperone and an inhibitor of apoptotic signaling. Four groups of experiments are proposed to test this hypothesis: Aim 1 is to determine if supporting cell Hsp70 is necessary and sufficient for the protective effect of Hsp70 against hair cell death. Aim 2 is to determine the roles of the anti-apoptotic and chaperone activities of Hsp70 in mediating its protective effect against hair cell death. Aim 3 is to determine if the protective effect of supporting cell Hsp70 is mediated by secretion of Hsp70 by supporting cells and internalization by hair cells. Aim 4 is to determine the intercellular signals that mediate hair cell-supporting cell communication in response to stress. From a clinical perspective, a clear understanding of the mechanisms underlying hair cell death and survival in response to ototoxic drugs will be critical to the rational design of co-therapies aimed at preventing hearing loss and balance disturbances caused by them. Our data indicate that Hsp70 is protective against hair cell death caused by both aminoglycosides and cisplatin, and thus it represents a potential therapeutic target for both classes of ototoxic drugs. From a basic science perspective, this project addresses the cellular biology of interactions between pro-survival and pro-death signaling in hair cells under stress. In addition, this research program examines the fundamental nature of the interactions between hair cells and supporting cells. PUBLIC HEALTH RELEVANCE: This project is focused on hearing loss caused by exposure to therapeutic drugs that damage the inner ears of at least half a million Americans every year. This project is designed to examine the mechanisms underlying the death and survival of sensory cells in the inner ear exposed to these drugs. Understanding these mechanisms will guide the development of therapies aimed at preventing hearing loss caused by ototoxic drugs.